Method of processing wafer

ABSTRACT

A method of processing a wafer includes a wafer preparing step of preparing a measurement wafer and a product wafer, a measurement etching step of supplying a gas in a plasma state to first areas of the measurement wafer that correspond to streets thereon to form grooves in the measurement wafer, a measuring step of demarcating a plurality of concentric areas in an array from a center to an outer circumference of the measurement wafer, and measuring depths of the grooves in the respective concentric areas, a thickness adjusting step of adjusting a thickness of the product wafer such that the product wafer is progressively thinner in areas thereof that correspond to the areas of the measurement wafer where the grooves are shallower, and an etching step of supplying a gas in a plasma state to second areas of the product wafer that correspond to streets thereon.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method of processing a wafer by wayof plasma etching.

Description of the Related Art

Processes of manufacturing device chips use wafers from which tofabricate device chips. A wafer has a plurality of devices formed inrespective areas demarcated on a face side thereof by a grid of streetsor projected dicing lines established on the face side. The wafer isdivided along the street into a plurality of device chips including therespective devices. The device chips will be incorporated in variouselectronic appliances such as mobile phones and personal computers.

For dividing a wafer, there is used a cutting apparatus for cutting thewafer with an annular cutting blade. When the cutting apparatus is inoperation, the annular cutting blade on the cutting apparatus is rotatedand cuts into the wafer along the streets, severing the wafer along thestreets. In recent years, attention has been given to the technology fordividing wafers according to a laser process. For example, a laser beamis applied to a wafer to form division initiating points such as groovesor modified layers in the wafer along streets established on the wafer.Then, external forces are applied to the wafer to cause the wafer tofracture from the division initiating points, thereby dividing the waferinto a plurality of device chips.

However, when a wafer is divided by a cutting process or a laserprocess, the wafer is liable to suffer processing defects. For example,when a wafer is divided by a cutting blade, the wafer may producechippings from its face or reverse side. When a wafer is divided by alaser process, the wafer may develop local regions where itscrystallinity is changed, i.e., crystal strain layers, surfaceirregularities, or the like due to the heat generated by a laser beamapplied to the wafer. If such processing defects remain on device chipsproduced from the wafer by dividing same, a problem arises in that thedevice chips have their flexural strength lowered.

In view of the above difficulties, there has been proposed a process ofdividing a wafer by way of plasma etching. For example, JP 2006-294686Adiscloses a method of dividing a wafer along streets by forming a resistfilm or mask on the reverse side of the wafer and supplying an etchinggas in a plasma state through the mask to the wafer. Plasma etching isless likely to afflict wafers with processing defects than the cuttingprocess and the laser process. Therefore, when a wafer is divided byplasma etching, the flexural strength of device chips produced from thewafer is restrained from being lowered.

SUMMARY OF THE INVENTION

For dividing a wafer by way of plasma etching, a gas in a plasma stateis supplied to an upper surface of the wafer, for example, forminggrooves in the wafer along streets established on the wafer. When thegrooves reach a lower surface of the wafer due to the etching inprogress, the wafer is divided along the streets. Therefore, etchingconditions including an etching time, a gas flow rate, etc. areestablished to cause the grooves to reach the lower surface of the waferall over the wafer. However, etching rate variations may occur in thewafer as a result of various factors including gas flow deviations,plasma density deviations, etc. in the plasma etching. In the event ofsuch etching rate variations, notwithstanding that the grooves in acentral area of the wafer have reached the lower surface of the wafer,the grooves in an outer circumferential portion of the wafer mayterminate short of the lower surface of the wafer, leading to imperfectdivision of the wafer. In view of the shortcoming, etching conditionsare adjusted for reliable division of the wafer to cause the grooves toreach the lower surface of the wafer in areas where the wafer is hardestto etch.

However, under the etching conditions thus adjusted, in the areas of thewafer where the etching rate is higher, i.e., where the etchingprogresses faster, the etching still continues after the grooves havereached the lower surface of the wafer, tending to excessively processthe lower surface of the wafer with the laser beam. As a consequence,some of the device chips produced by dividing the wafer tend to bebrought out of shape, resulting in a reduction in the quality of thedevice chips.

The present invention has been made in view of the above problems. It istherefore an object of the present invention to provide a method ofprocessing a wafer to appropriately divide the wafer by way of plasmaetching.

In accordance with an aspect of the present invention, there is provideda method of processing a wafer, including: a wafer preparing step ofpreparing a measurement wafer and a product wafer each including a firstsurface that has a plurality of areas demarcated by a plurality ofstreets thereon and a second surface that is opposite the first surface;a measurement etching step of forming a first mask on the first surfaceor the second surface of the measurement wafer and supplying a gas in aplasma state to first areas of the measurement wafer that are exposedthrough the first mask and that correspond to the streets to etch thefirst areas under predetermined conditions to form grooves in themeasurement wafer; after the measurement etching step, a measuring stepof demarcating a plurality of concentric areas in an array from a centerto an outer circumference of the measurement wafer and measuring depthsof the grooves in the respective concentric areas; after the measuringstep, a thickness adjusting step of adjusting a thickness of the productwafer such that the product wafer is progressively thinner in areasthereof that correspond to the areas of the measurement wafer where thegrooves are shallower; and, after the thickness adjusting step, anetching step of forming a second mask on the first surface or the secondsurface of the product wafer and supplying a gas in a plasma state tosecond areas of the product wafer that are exposed through the secondmask and that correspond to the streets to etch the second areas underpredetermined conditions.

Preferably, the thickness adjusting step includes performing grinding,polishing, or plasma etching on the product wafer to adjust thethickness of the product wafer. Preferably, the measurement etching stepincludes supplying the gas in the plasma state to the measurement waferwhile a first protective member for protecting the measurement wafer isdisposed on a surface of the measurement wafer that is opposite to thesurface thereof to which the gas in the plasma state is supplied, andthe etching step includes supplying the gas in the plasma state to theproduct wafer while a second protective member for protecting theproduct wafer is disposed on a surface of the product wafer that isopposite to the surface thereof to which the gas in the plasma state issupplied.

In the method of processing a wafer according to the aspect of thepresent invention, the thickness of the product wafer is adjusteddepending on the depths of the grooves formed in the measurement waferby plasma etching, and thereafter plasma etching is performed on theproduct wafer. The etching rate variations in the product wafer are thusreflected in the thickness distribution of the product wafer, therebysynchronizing the times when the division of the product wafer in theareas thereon is completed. In this manner, the product wafer is lessliable to have areas where it is etched imperfectly and areas where itis etched excessively, and hence can be divided properly.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of a measurement wafer;

FIG. 1B is a perspective view of a product wafer;

FIG. 2A is a perspective view of the measurement wafer with a protectivemember attached thereto;

FIG. 2B is a perspective view of the product wafer with a protectivemember attached thereto;

FIG. 3A is an enlarged fragmentary cross-sectional view of themeasurement wafer with a mask layer formed thereon;

FIG. 3B is an enlarged fragmentary cross-sectional view of themeasurement wafer with a mask formed thereon;

FIG. 4 is a schematic view, partly in cross section, of a plasmaprocessing apparatus;

FIG. 5 is an enlarged fragmentary cross-sectional view of themeasurement wafer to which a gas in a plasma state is supplied;

FIG. 6A is an enlarged fragmentary cross-sectional view illustrating acentral portion of the measurement wafer after plasma etching;

FIG. 6B is an enlarged fragmentary cross-sectional view illustrating anouter circumferential portion of the measurement wafer after plasmaetching;

FIG. 7 is a cross-sectional view of the measurement wafer with groovesformed therein;

FIG. 8 is a front elevational view, partly in cross section, of agrinding apparatus;

FIG. 9 is a cross-sectional view of a chuck table of the grindingapparatus;

FIG. 10A is a cross-sectional view of a product wafer that has beenground on the chuck table tilted in a first direction;

FIG. 10B is a cross-sectional view of a product wafer that has beenground on the chuck table tilted in a second direction;

FIG. 11A is an enlarged fragmentary cross-sectional view of the productwafer in an etching step; and

FIG. 11B is an enlarged fragmentary cross-sectional view of the productwafer after plasma etching.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will be described belowwith reference to the accompanying drawings. First, configurationexamples of wafers that can be used in a method of processing a waferaccording to the present embodiment will be described below. Accordingto the present embodiment, the wafers that can be used in the method ofprocessing a wafer include a product wafer to be used in manufacturingactual products and a measurement wafer to be used in selectingprocessing conditions for the product wafer.

FIG. 1A illustrates in perspective a wafer 11 that servers as ameasurement wafer, also referred to as a test wafer, to be used inselecting processing conditions for a product wafer (see FIG. 1B). Thewafer 11 includes a disk-shaped substrate made of a semiconductor suchas silicon, for example, and includes a face side, i.e., a firstsurface, 11 a and a reverse side, i.e., a second surface, 11 b oppositethe face side 11 a. The face side 11 a of the wafer 11 has a pluralityof rectangular areas demarcated by a grid of streets or projected dicinglines 13 established on the face side 11 a. The streets 13 are separatedinto two groups that extend across each other. A plurality of devices 15such as integrated circuits (ICs), large-scale-integration (LSI)circuits, light-emitting diodes (LEDs), or microelectromechanicalsystems (MEMS) are formed respectively in the demarcated areas.

The wafer 11 is not limited to any particular material, shape,structure, size, etc. For example, the wafer 11 may be a substrate madeof semiconductors other than silicon, e.g., GaAs, InP, GaN, SiC, or thelike, sapphire, glass, ceramic, resin, metal, or the like. The devices15 are not limited to any kind, quantity, shape, structure, size,layout, etc.

FIG. 1B illustrates in perspective a wafer 31 that serves as a productwafer to be used in manufacturing actual products (device chips or thelike). The wafer 31 has a configuration similar to that of the wafer 11(see FIG. 1A). Specifically, the wafer 31 includes a face side, i.e., afirst surface, 31 a and a reverse side, i.e., a second surface, 31 bopposite the face side 31 a. The face side 31 a of the wafer 31 has aplurality of rectangular areas demarcated by a grid of streets orprojected dicing lines 33 established on the face side 31 a. The streets33 are separated into two groups that extend across each other. Aplurality of devices 35 are formed respectively in the demarcated areas.The wafer 31 is of the similar material, shape, structure, size, etc. tothe wafer 11. The devices 35 are of the similar kind, quantity, shape,structure, size, layout, etc. to the devices 15.

The wafer 31 will be processed and cut along the streets 33 into aplurality of device chips including the respective devices 35.Processing conditions for processing the wafer 31 are selected on thebasis of the results of a test conducted using the wafer 11. A specificexample of the method of processing a wafer according to the presentembodiment will be described below.

First, a measurement wafer and a product wafer are prepared (waferpreparing step). In the wafer preparing step, specifically, the wafer 11illustrated in FIG. 1A and the wafer 31 illustrated in FIG. 1B areformed.

Since the wafer 11 is used in selecting processing conditions forprocessing the wafer 31, as described later, the wafer 11 shouldpreferably have similar details to the wafer 31. Specifically, the wafer11 should preferably be made of the same material as the wafer 31.Further, the number of the streets 13 should preferably be the same asthe number of the streets 33, and the width of the streets 13 shouldpreferably be generally the same as the width of the streets 33.Further, the dimensions of and distances between the devices 15 on thewafer 11 should preferably be generally the same as the dimensions ofand distances between the devices 35 on the wafer 31. However, thedetails of the wafer 11 may be different from the details of the wafer31 within a range where processing conditions for processing the wafer31 can appropriately be selected. For example, since the wafer 11 is notused in manufacturing actual device chips, the wafer 11 may notnecessarily have the devices 15.

The wafer 11 and the wafer 31 will be processed by plasma etching in asubsequent step. Therefore, a protective member for protecting the wafer11 is applied to the face side 11 a or the reverse side 11 b of thewafer 11. Similarly, a protective member for protecting the wafer 31 isapplied to the face side 31 a or the reverse side 31 b of the wafer 31.

FIG. 2A illustrates in perspective the wafer 11 with a protectivemember, i.e., a first protective member, 17 disposed thereon. Forexample, a circular protective member 17 that is larger in diameter thanthe wafer 11 is fixed to the face side 11 a of the wafer 11. The faceside 11 a of the wafer 11 and the devices 15 thereon are thus coveredwith and protected by the protective member 17. The protective member 17may include a film-like tape, for example. The tape includes a circularbase and an adhesive layer, i.e., a glue layer, disposed on the base.The base is made of a resin such as polyolefin, polyvinyl chloride, orpolyethylene terephthalate, for example, whereas the adhesive layer ismade of an adhesive of an epoxy material, an acrylic material, a rubbermaterial, or the like, for example. Alternatively, the adhesive layermay be made of an ultraviolet-curable resin that can be cured byultraviolet rays applied thereto.

The protective member 17 has an outer circumferential portion affixed toan annular frame 19 made of metal or the like. The frame 19 has acircular opening 19 a defined therein that is larger in diameter thanthe wafer 11. The wafer 11 is affixed centrally to the protective member17 such that the wafer 11 is disposed within the opening 19 a. The wafer11 is thus supported on the frame 19 by the protective member 17 forbeing easily handled, e.g., transported, held, or otherwise treated.

FIG. 2B illustrates in perspective the wafer 31 with a protectivemember, i.e., a second protective member, 37 disposed thereon. Forexample, a circular protective member 37 that is larger in diameter thanthe wafer 31 is fixed to the face side 31 a of the wafer 31. The faceside 31 a of the wafer 31 and the devices 35 thereon are thus coveredwith and protected by the protective member 37. The protective member 37has an outer circumferential portion affixed to an annular frame 39 madeof metal or the like. The frame 39 has a circular opening 39 a definedtherein that is larger in diameter than the wafer 31. The wafer 31 isaffixed centrally to the protective member 37 such that the wafer 31 isdisposed within the opening 39 a. The wafer 31 is thus supported on theframe 39 by the protective member 37. The protective member 37 is of thesimilar shape, structure, material, etc. to the protective member 17.The frame 39 is of the similar shape, structure, material, etc. to theframe 19.

The wafer 11 and the wafer 31 may be formed at appropriately selectedtimes. Specifically, the wafer 11 may be prepared until a measurementetching step to be described later, and the wafer 31 may be prepareduntil a thickness adjusting step to be described later.

Next, a gas in a plasma state is supplied to the areas of the wafer 11that correspond to the streets 13 to form grooves in the wafer 11(measurement etching step). In the measurement etching step, the wafer11 is processed by plasma etching to etch the wafer 11 along the streets13. By way of example, a process of supplying a gas in a plasma state tothe reverse side 11 b of the wafer 11 to etch the wafer 11 will bedescribed below as the measurement etching step.

In the measurement etching step, first, a mask 23 (see FIG. 3B) forplasma etching is formed on the reverse side 11 b of the wafer 11. Forexample, the mask 23 may be formed on the reverse side 11 b bydepositing a mask layer 21 (see FIG. 3A) on the reverse side 11 b andthen removing the areas of the mask layer 21 that correspond to thestreets 13.

FIG. 3A illustrates in enlarged fragmentary cross section the wafer 11with the mask layer 21 formed thereon. The mask layer 21 is made of amaterial functioning as a mask during plasma etching and is deposited incovering relation to the reverse side 11 b in its entirety. For example,the mask layer 21 may be made of a water-soluble resin such as polyvinylalcohol (PVA) or polyethylene glycol (PEG). Then, the areas of the masklayer 21 that correspond to the streets 13 are removed. For example, alaser beam is applied to the mask layer 21 along the streets 13 toremove the areas of the mask layer 21 that correspond to the streets 13.The laser beam is applied under conditions including a wavelength, apower level, a spot diameter, a repetitive frequency, etc. that areselected to process the mask layer 21 by way of ablation when the laserbeam is applied to the mask layer 21. When the mask layer 21 is removedalong all the streets 13, a grid-shaped opening through which part ofthe reverse side 11 b of the wafer 11 is exposed is formed in the masklayer 21.

FIG. 3B illustrates in enlarged fragmentary cross section the wafer 11with the mask, i.e., a first mask, 23 formed thereon. The mask layer 21is patterned as described above to form the mask 23 through which theareas of the reverse side 11 b of the wafer 11 that correspond to thestreets 13, i.e., the areas overlapping the streets 13, are exposed andwhich covers the areas of the reverse side 11 b of the wafer 11 thatcorrespond to the devices 15, i.e., the areas overlapping the devices15. The mask 23 may be formed of other materials than the materialsdescribed above and may be formed by other processes than the processdescribed above. For example, the mask layer 21 may be made of a resistof photosensitive resin and may be patterned into the mask 23 by beingexposed to light.

Next, a gas in a plasma state is supplied to the areas of the wafer 11that are exposed through the mask 23 and correspond to the streets 13,etching those areas under predetermined conditions. The wafer 11 isetched using a plasma processing apparatus, for example. FIG. 4schematically illustrates in cross section a plasma processing apparatus10 that is used to etch the wafer 11.

As illustrated in FIG. 4 , the plasma processing apparatus 10 includes achamber 12 in the shape of a rectangular parallelepiped. The chamber 12includes a bottom wall 12 a, an upper wall 12 b, a first side wall 12 c,a second side wall 12 d, a third side wall 12 e, and a fourth side wall,not illustrated. The chamber 12 has an internal space acting as aprocessing space 14 in which a plasma process is carried out.

The second side wall 12 d has an opening 16 defined therein throughwhich a wafer 11 can be loaded into and unloaded out of the processingspace 14. A gate, i.e., an openable and closable door, 18 for openingand closing the opening 16 is disposed outside of the opening 16. Thegate 18 is connected to an opening and closing mechanism 20 that canmove the gate 18 vertically, i.e., upwardly and downwardly. The openingand closing mechanism 20 includes an air cylinder 22 having a piston rod24, for example. The piston rod 24 has an upper end coupled to a lowerportion of the gate 18. The air cylinder 22 is fixed to the bottom wall12 a of the chamber 12 by a bracket 26. When the opening and closingmechanism 20 lowers the gate 18, the opening 16 is exposed. Now, a wafer11 can be loaded through the opening 16 into the processing space 14 orunloaded out of the processing space 14 through the opening 16.

The bottom wall 12 a of the chamber 12 has an exhaust port 28 definedtherein that provides fluid communication between the inside and outsideof the chamber 12. An exhaust mechanism 30 for evacuating the processingspace 14 is connected to the exhaust port 28. The exhaust mechanism 30includes a vacuum pump, for example.

The processing space 14 houses therein a lower electrode 32 and an upperelectrode 34 that vertically confront each other. The lower electrode 32is made of an electrically conductive material and includes adisk-shaped holder 36 and a cylindrical support post 38 projectingdownwardly from a central portion of a lower surface of the holder 36.

The support post 38 is inserted through an opening 40 defined in thebottom wall 12 a of the chamber 12. An annular insulating member 42 isdisposed in the opening 40 between the bottom wall 12 a and the supportpost 38, insulating the chamber 12 and the lower electrode 32 from eachother. The lower electrode 32 is electrically connected to ahigh-frequency power supply 44 disposed outside of the chamber 12. Theholder 36 has an upwardly open recess defined in an upper surfacethereof and housing therein a disk-shaped table 46 for holding the wafer11 thereon. The table 46 has an upper surface acting as a holdingsurface 46 a for holding the wafer 11 thereon. The holding surface 46 ais connected to a suction source 50 such as an ejector through a fluidchannel, not illustrated, defined in the table 46 and a fluid channel 48defined in the lower electrode 32. The holder 36 has a coolant channel52 defined therein. The coolant channel 52 has an end connected to acoolant circulating mechanism 56 through a coolant inlet passage 54defined in the support post 38. The other end of the coolant channel 52is connected to the coolant circulating mechanism 56 through a coolantoutlet passage 58 defined in the support post 38. When the coolantcirculating mechanism 56 is actuated, a coolant flows therefromsuccessively through the coolant inlet passage 54, the coolant channel52, and the coolant outlet passage 58, cooling the lower electrode 32.

The upper electrode 34 is made of an electrically conductive materialand includes a disk-shaped gas ejecting member 60 and a cylindricalsupport post 62 projecting upwardly from a central portion of an uppersurface of the gas ejecting member 60. The support post 62 is insertedthrough an opening 64 defined in the upper wall 12 b of the chamber 12.An annular insulating member 66 is disposed in the opening 64 betweenthe upper wall 12 b and the support post 62, insulating the chamber 12and the upper electrode 34 from each other. The upper electrode 34 iselectrically connected to a high-frequency power supply 68 disposedoutside of the chamber 12. The support post 62 has an upper end on whichthere is mounted a support arm 72 coupled to a lifting and loweringmechanism 70. The lifting and lowering mechanism 70 and the support arm72 move the upper electrode 34 vertically, i.e., lift the upperelectrode 34 upwardly and lower the upper electrode 34 downwardly. Thegas ejecting member 60 has a plurality of ejection ports 74 definedtherein and opening downwardly from a lower surface thereof. Theejection ports 74 are connected to a first gas supply source 80 and asecond gas supply source 82 through a fluid channel 76 defined in thegas ejecting member 60 and a fluid channel 78 defined in the supportpost 62. The first gas supply source 80 and the second gas supply source82 are capable of supplying respective gases that contain differentcomponents to the fluid channel 78.

The constituents of the plasma processing apparatus 10 that include theopening and closing mechanism 20, the exhaust mechanism 30, thehigh-frequency power supply 44, the suction source 50, the coolantcirculating mechanism 56, the high-frequency power supply 68, thelifting and lowering mechanism 70, the first gas supply source 80, andthe second gas supply source 82, etc. are electrically connected to acontroller, i.e., a control unit or a control device, 84 that controlsthe plasma processing apparatus 10. The controller 84 controls operationof the constituents of the plasma processing apparatus 10. For example,the controller 84 includes a computer and includes a processing sectionfor carrying out processing operations required to operate the plasmaprocessing apparatus 10 and a storage section for storing various piecesof information including data, programs, etc. used by the processingsection in carrying out processing operations. The processing sectionincludes a processor such as a central processing unit (CPU). Thestorage section includes various memories acting as a main storagedevice, an auxiliary storage device, etc. The processing sectiongenerates control signals for controlling the constituents of the plasmaprocessing apparatus 10 by executing the programs stored in the storagesection.

For performing plasma etching on a wafer 11 with the plasma processingapparatus 10, the opening and closing mechanism 20 lowers the gate 18 ofthe plasma processing apparatus 10 to expose the opening 16. Then, adelivery mechanism, not illustrated, loads the wafer 11 through theopening 16 into the processing space 14 in the chamber 12 and places thewafer 11 on the table 46. At this time, the wafer 11 is placed on thetable 46 with the reverse side 11 b, i.e., the mask 23 side, exposedupwardly toward the upper electrode 34. When the wafer 11 is to beloaded into the processing space 14, it is preferable for the liftingand lowering mechanism 70 to lift the upper electrode 34, increasing thespacing between the lower electrode 32 and the upper electrode 34.

Next, the suction source 50 applies a negative pressure to the holdingsurface 46 a of the table 46, holding the wafer 11 under suction on theholding surface 46 a. The opening and closing mechanism 20 lifts thegate 18 to close the opening 16, hermetically sealing the processingspace 14. The lifting and lowering mechanism 70 adjusts the verticalposition of the upper electrode 34 to bring the upper electrode 34 andthe lower electrode 32 into a predetermined positional relation suitablefor a plasma process. Then, the exhaust mechanism 30 is actuated toevacuate the processing space 14 to a reduced pressure ranging from 50to 300 Pa, for example. If the negative pressure applied from thesuction source 50 fails to keep the wafer 11 under suction on the table46 when the processing space 14 is evacuated, then the wafer 11 is heldon the table 46 under electric forces, typically electrostatic forces.For example, a plurality of electrodes are embedded in the table 46. Apredetermined voltage is applied to these electrodes to apply a Coulombforce between the table 46 and the wafer 11, thereby attracting thewafer 11 to the table 46. At this time, the table 46 functions as anelectrostatic chuck table.

Then, the first gas supply source 80 or the second gas supply source 82supplies a gas for etching, i.e., an etching gas, between the lowerelectrode 32 and the upper electrode 34 through the fluid channel 78,the fluid channel 76, and the ejection ports 74. At the same time, apredetermined level of high-frequency electric power ranging from 1000to 3000 W, for example, is applied between the lower electrode 32 andthe upper electrode 34. As a result, the gas existing between the lowerelectrode 32 and the upper electrode 34 turns into a plasma statecontaining ions and radicals. The gas in the plasma state is supplied tothe reverse side 11 b of the wafer 11.

FIG. 5 illustrates in enlarged fragmentary cross section the wafer 11 towhich the gas, denoted by 90, in the plasma state is supplied. The gas90 in the plasma state is specifically supplied to those areas, i.e.,first areas, 11 c of the reverse side 11 b of the wafer 11 that are notcovered with the mask 23. The areas 11 c are exposed through the mask 23and correspond to the areas corresponding to the streets 13, i.e., theareas overlapping the streets 13. As a result, plasma etching isperformed on the areas 11 c. In the measurement etching step, the gas 90is supplied to the wafer 11 while the protective member 17 is disposedon the surface, i.e., the face side 11 a, of the wafer 11 that isopposite the surface thereof, i.e., the reverse side 11 b, to which thegas 90 is supplied. The components of the gas 90 are appropriatelyselected depending on the material of the wafer 11. For example, if thewafer 11 is a silicon wafer, then the gas 90 contains a fluorine gassuch as CF₄ or SF₆.

FIG. 6A illustrates in enlarged fragmentary cross section a centralportion of the wafer 11 after the plasma etching, and FIG. 6Billustrates in enlarged fragmentary cross section an outercircumferential portion of the wafer 11 after the plasma etching. Whenthe gas 90 in the plasma state is supplied through the mask 23 to thereverse side 11 b of the wafer 11, the areas 11 c are etched, forminggrooves 11 d in the wafer 11 from the reverse side 11 b toward the faceside 11 a.

Plasma etching conditions including an etching time, a gas flow rate, ahigh-frequency electric power level, a pressure in the processing space14, etc. are identical to plasma etching conditions for dividing thewafer 31 at a subsequent step (see FIGS. 11A and 11B), for example. Thewafer 11 is thicker than the wafer 31. As a result, the grooves 11 dthat terminate short of the face side 11 a are formed in the wafer 11from the reverse side 11 b along the streets 13.

When the plasma etching is performed on the wafer 11 by the plasmaprocessing apparatus 10, etching rate variations may occur in the wafer11 as a result of various factors including gas flow deviations, plasmadensity deviations, etc. For example, etching tends to progress fasterin the central portion of the wafer 11, whereas etching is liable toprogress slower in the outer circumferential portion of the wafer 11. Inthe event of such an etching rate variation, a groove 11 d (see FIG. 6B)formed in the outer circumferential portion of the wafer 11 is shallowerthan a groove 11 d (see FIG. 6A) formed in the central portion of thewafer 11.

After the plasma etching on the wafer 11 has been completed, the mask 23is removed from the wafer 11. In a case where the mask 23 is made of awater-soluble resin, the mask 23 can easily be removed by supplying thereverse side 11 b of the wafer 11 with pure water or the like.

Next, a plurality of concentric areas are demarcated on the wafer 11 ina radial array from the center to the outer circumference of the wafer11, and the depths of the grooves 11 d are measured in the respectiveconcentric areas (measuring step). In the measuring step, the wafer 11with the grooves 11 d formed in a grid pattern along the streets 13 isused.

FIG. 7 illustrates in cross section the wafer 11 with the grooves 11 dformed therein. If etching rate variations occur in the wafer 11 in themeasurement etching step, then the grooves 11 d formed in the wafer 11have different depths. For example, as illustrated in FIG. 7 , thegrooves 11 d are progressively deeper in those areas that are closer tothe center of the wafer 11 and progressively shallower in those areasthat are closer to the outer circumference of the wafer 11.

In the measuring step, the wafer 11 after the measurement etching stephas been carried out is cut along one of the streets 13, thereby makinga cross section of the wafer 11 observable as illustrated in FIG. 7 .Then, a plurality of areas are demarcated on the wafer 11 in a radialarray from the center to the outer circumference of the wafer 11. Forexample, a circular area A₁ including the center of the wafer 11 and aplurality of annular areas A₂, A₃, A₄, and A₅ around the area A₁ aredemarcated on the wafer 11. The areas A₁, A₂, A₃, A₄, and A₅ areconcentric with each other and have progressively larger diameters. Theradius of the area A₁ and the widths of the areas A₂, A₃, A₄, and A₅ aregenerally the same as each other, for example. There is no limitation onthe number and diameters or widths of the areas demarcated on the wafer11. In other words, the area A₁ and any number of annular areas disposedaround the area A₁ and having different diameters are demarcated on thewafer 11.

Next, the depths of the grooves 11 d are measured for the respectiveareas A₁ through A₅. For example, an image of the cross section of thewafer 11 is captured and the depths of the grooves 11 d included in thecaptured image are actually measured. In a case where a plurality ofgrooves 11 d are included in one area, the depth of any one of thegrooves 11 d may be measured or the depths of the grooves 11 d in thearea may be measured and their average value may be calculated. Then,the depths of the grooves 11 d for the respective areas are recorded.

The depths of the grooves 11 d are commensurate with etching rates forthe grooves 11 d at the time the plasma processing apparatus 10 (seeFIG. 4 ) performs plasma etching on the wafer 11. In other words, in anarea where the groove 11 d is deeper, the plasma etching tends toprogress faster and the etching rate is higher. Therefore, the measuringstep that is carried out as described above confirms a distribution ofetching rate variations in the wafer 11.

Then, a product wafer is processed by plasma etching. For example, thewafer 31 (see FIG. 2B) is etched along the streets 33 and divided into aplurality of device chips including the respective devices 35. The wafer31 is etched using the plasma processing apparatus 10 (see FIG. 4 ).However, if there are etching rate variations in the wafer 11 at thetime plasma etching is performed on the wafer 11, the wafer 31 will beunlikely to be divided properly. For example, in a case where theetching rate on the outer circumferential portion of the wafer 31 islower than the etching rate on the central portion of the wafer 31 (seeFIGS. 6A and 6B), the outer circumferential portion of the wafer 31 willbe likely to be divided imperfectly. If processing conditions are variedto make sure that the outer circumferential portion of the wafer 31 willbe divided completely, then the plasma etching still continues in thecentral area of the wafer 31 after the division has been completed,thereby excessively processing the wafer 11 to the extent that devicechips produced from the central area of the wafer 31 may possibly suffera reduction in quality.

According to the present embodiment, the thickness of the wafer 31 isadjusted on the basis of the measured results, i.e., the measuredthicknesses, from the measuring step (thickness adjusting step).Specifically, before the plasma etching is carried out, the wafer 31 isprocessed to adjust the thickness thereof such that the wafer 31 isthicker in areas where the etching rate is higher, i.e., areas whereetching tends to progress faster, and thinner in areas where the etchingrate is lower, i.e., areas where etching tends to progress slower.

In the thickness adjusting step, first, a plurality of concentric areasare demarcated on the wafer 31 in a radial array from the center to theouter circumference of the wafer 31. Specifically, as with the wafer 11(see FIG. 7 ), areas A₁ through A₅ are demarcated on the wafer 31. Thedimensions, i.e., the diameters and widths, of the areas A₁ through A₅demarcated on the wafer 31 are identical to the dimensions of the areasA₁ through A₅ demarcated on the wafer 11.

Next, the thickness of the wafer 31 is adjusted such that those areas ofthe wafer 31 that correspond to the areas of the wafer 11 where thedepths of the grooves 11 d (see FIG. 7 ) measured in the measuring stepare smaller, i.e., the grooves 11 d are shallower, are thinner.Specifically, as illustrated in FIG. 7 , the grooves 11 d are deeper inthose areas of the wafer 11 that are closer to the center of the wafer11, and shallower in those areas of the wafer 11 that are closer to theouter circumference of the wafer 11. Accordingly, the wafer 31 isprocessed to become progressively thinner from the center, i.e., thearea A₁, toward the outer circumference, i.e., the area A₅.

The thickness of the wafer 31 is adjusted using a grinding apparatus,for example. FIG. 8 illustrates a grinding apparatus 100 in frontelevation, partly in cross section, that can be used in the thicknessadjusting step. As illustrated in FIG. 8 , the grinding apparatus 100includes a chuck table, i.e., a holding table, 102 for holding the wafer31 thereon and a grinding unit 106 for grinding the wafer 31 held on thechuck table 102.

The chuck table 102 has an upper surface acting as a flat holdingsurface 102 a for holding the wafer 31 thereon. The holding surface 102a is connected to a suction source, not illustrated, such as an ejectorthrough a fluid channel 102 b (see FIG. 9 ) defined in the chuck table102, a valve, not illustrated, etc. To the chuck table 102, there areconnected a ball-screw moving mechanism, not illustrated, for moving thechuck table 102 in horizontal directions and a rotary actuator, notillustrated, such as an electric motor for rotating the chuck table 102about a rotational axis generally parallel to vertical directions. Thegrinding apparatus 100 also includes a plurality of clamps 104 disposedaround the chuck table 102 for gripping and securing a frame 39 thatsupports the wafer 31.

The grinding unit 106 is disposed above the chuck table 102. Thegrinding unit 106 includes a hollow cylindrical spindle 108 extending invertical directions. A disk-shaped wheel mount 110 is fixed to thedistal end, i.e., the lower end, of the spindle 108. A rotary actuator,not illustrated, such as an electric motor for rotating the spindle 108about its central axis is connected to the proximal end, i.e., the upperend, of the spindle 108.

A grinding wheel 112 for grinding the wafer 31 is mounted on a lowersurface of the wheel mount 110. The grinding wheel 112 includes anannular base 114 made of a metal such as stainless steel or aluminum andhaving substantially the same diameter as the wheel mount 110. Thegrinding wheel 112 also includes a plurality of grindstones 116 fixed toa lower surface of the base 114. Each of the grindstones 116 is shapedas a rectangular parallelepiped. The grindstones 116 are arrayed in anannular pattern at generally equal spaced intervals along the outercircumferential edge of the base 114. The grinding wheel 112 isrotatable about a rotational axis generally parallel to verticaldirections by rotational power transmitted from the rotary actuatorthrough the spindle 108 and the wheel mount 110. A ball-screw movingmechanism, not illustrated, is connected to the grinding unit 106 forlifting and lowering the grinding unit 106 in vertical directions. Anozzle 118 for supplying a grinding fluid 120 such as pure water to thewafer 31 held on the chuck table 102 and the grindstones 116 of thegrinding wheel 112 is disposed in the vicinity of the grinding unit 106.

For grinding the wafer 31, the wafer 31 is held on the chuck table 102.Specifically, the wafer 31 is placed on the chuck table 102 such thatthe face side 31 a, i.e., the protective member 37 side, faces theholding surface 102 a and the reverse side 31 b is exposed upwardly. Theframe 39 is gripped and secured in position by the clamps 104. Then, thesuction source applies a negative pressure to the holding surface 102 aof the chuck table 102, holding the wafer 31 under suction on theholding surface 106 a with the protective member 37 interposedtherebetween.

Next, the chuck table 102 is moved to a position below the grinding unit106. Then, while the chuck table 102 and the grinding wheel 112 arebeing rotated about their rotational axes at respective predeterminedrotational speeds in respective directions, the grinding wheel 112 islowered toward the chuck table 102. The speed at which the grindingwheel 112 is lowered is adjusted such that the grindstones 116 arepressed against the wafer 31 under appropriate forces. When the rotatinggrindstones 116 are brought into contact with the reverse side 31 b ofthe wafer 31, the grindstones 116 start scraping the reverse side 31 bof the wafer 31, thereby grinding and thinning the wafer 31. While thewafer 31 is thus ground, the grinding fluid 120 supplied from the nozzle118 cools the wafer 31 and the grindstones 116 and washes away debris,i.e., swarf, produced by the grinding of the wafer 31. The wafer 31 iscontinuously ground until it is thinned to a predetermined thickness,i.e., a finished thickness, whereupon the wafer 31 stops being ground.

When the wafer 31 is ground by the grinding apparatus 100, the shape ofthe wafer 31 that will have been ground can be controlled by adjustingthe angle of the chuck table 102. FIG. 9 illustrates the chuck table 102in cross section.

The chuck table 102 has an upwardly open circular recess defined in anupper surface thereof, and a disk-shaped holder, i.e., a suction member,102 c made of a porous material such as porous ceramic is fitted in thecircular recess. The holder 102 c is connected to a suction source, notillustrated, through the fluid channel 102 b defined in the chuck table102. The holder 102 c has an upper surface acting as a holding surface102 a of the chuck table 102. The holder 102 c has its thicknessprogressively larger from its outer circumferential edge toward itscenter. Specifically, the upper surface, i.e., the holding surface 102a, of the holder 102 c is of an upwardly projected V-shaped crosssection with the crest at its center. In FIG. 9 , the gradient of theholding surface 102 a is illustrated as exaggerated. If the holder 102 chas a diameter ranging from approximately 290 to 310 mm, for example,then the difference between the vertical position of the center of theupper surface of the holder 102 c, i.e., the position of the center ofthe upper surface thicknesswise of the holder 102 c, and the verticalposition of the outer circumferential edge of the upper surface of theholder 102 c is set to a value in a range of approximately 10 to 20 μm.

In the grinding apparatus 100, the chuck table 102 is installed in aslightly tilted state such that an area 102 d of the holding surface 102a that underlies the grindstones 116 (see FIG. 8 ) lies parallel to thelower surfaces of the grindstones 116 that lie in horizontal directions.The chuck table 102 is rotatable about a rotational axis generallyparallel to the thicknesswise directions of the holder 102 c. Theposition of the center of the holder 102 c is aligned with the positionof the rotational axis about which the chuck table 102 is rotatable. Thewafer 31 is held under suction on the chuck table 102 in a slightlycurved state along the holding surface 102 a, and will be ground by thegrindstones 116.

The chuck table 102 is arranged to make its angle of tilt variable.Specifically, the chuck table 102 can be tilted to incline itsrotational axis in a first direction, i.e., the direction indicated byan arrow B, and a second direction, i.e., the direction indicated by anarrow C. The shape of the wafer 31 that will have been ground can becontrolled by adjusting the tilt of the chuck table 102.

FIG. 10A illustrates in cross section a wafer 31 that has been ground onthe chuck table 102 tilted in the first direction, and FIG. 10Billustrates in cross section a wafer 31 that has been ground on thechuck table 102 tilted in the second direction. In a case where thechuck table 102 is tilted in the first direction (see the arrow B inFIG. 9 ) and the wafer 31 is ground on the chuck table 102 where thecenter of the holding surface 102 a is slightly lower than the outercircumferential edge thereof in the area 102 d, the wafer 31 has itsouter circumferential portion ground preferentially. As a result, asillustrated in FIG. 10A, the ground wafer 31 is ground such that it isprogressively thicker from the outer circumferential edge toward thecenter thereof, with the reverse side 31 b being upwardly projected. Onthe other hand, in a case where the chuck table 102 is tilted in thesecond direction (see the arrow C in FIG. 9 ) and the wafer 31 is groundon the chuck table 102 where the center of the holding surface 102 a isslightly higher than the outer circumferential edge thereof in the area102 d, the wafer 31 has its central portion ground preferentially. As aresult, as illustrated in FIG. 10B, the ground wafer 31 is ground suchthat it is progressively thinner from the outer circumferential edgetoward the center thereof, with the reverse side 31 b being downwardlyprojected.

In the thickness adjusting step, the wafer 31 is ground such that it isthinner in those areas of the wafer 31 that correspond to the areas ofthe wafer 11 where the grooves 11 d (see FIG. 7 ) are shallower, andthicker in those areas of the wafer 31 that correspond to the areas ofthe wafer 11 where the grooves 11 d are deeper. Specifically, in thewafer 11, the etching rate is lower and the grooves 11 d are shallowerin those areas that are closer to the outer circumferential edgethereof. Further, the wafer 31 is made of a material identical orsimilar to the material of the wafer 11, and the distribution of etchingrate variations of the wafer 31 exhibits the same tendency as thedistribution of etching rate variations of the wafer 11. Accordingly,the chuck table 102 of the grinding apparatus 100 is tilted in the firstdirection (see the arrow B in FIG. 9 ) and the wafer 31 is ground on thechuck table 102 thus tilted. In this manner, the wafer 31 that isthicker in the areas, i.e., the central portion, where the etching rateis higher and thinner in the areas, i.e. the outer circumferentialportion, where the etching rate is lower (see FIG. 10A).

There is no limitation on processes of adjusting the thickness of thewafer 31. For example, the thickness of the wafer 31 may be adjustedusing a polishing apparatus in place of the grinding apparatus 100. Thepolishing apparatus includes a chuck table, i.e., a holding table, forholding the wafer 31 and a polishing unit with a polishing pad mountedthereon for polishing the wafer 31 held on the chuck table. Thepolishing pad includes, for example, a disk-shaped polishing layer madeof nonwoven fabric or foamed urethane with abrasive grains, i.e., fixedabrasive grains, dispersed therein. The abrasive grains of the polishinglayer may be made of silica whose particle diameters range fromapproximately 0.1 to 10 μm. While supplying the wafer 31 and thepolishing pad with a polishing fluid, the chuck table and the polishingpad are rotated about their respective rotational axes and the polishinglayer of the polishing pad is pressed against the wafer 31, therebypolishing the wafer 31. As is the case with the grinding apparatus 100,the angle of tilt of the chuck table of the polishing apparatus can beadjusted (see FIG. 9 ) to control the thickness of the wafer 31 (seeFIGS. 10A and 10B). In the thickness adjusting step, alternatively, thethickness of the wafer 31 may be adjusted by etching the wafer 31 in itsentirety using the plasma processing apparatus 10 (see FIG. 4 ). In thiscase, the wafer 31 may be processed to a desired shape by appropriatelyadjusting the position of the wafer 31 and etching conditions for plasmaetching.

Next, a gas in a plasma state is supplied to the areas of the wafer 31that correspond to the streets 33, thereby etching the wafer 31 (etchingstep). FIG. 11A illustrates the wafer 31 in the etching step in enlargedfragmentary cross section. A process of etching the wafer 31 bysupplying a gas in a plasma state to the reverse side 31 b of the wafer31 will be described below by way of example.

In the etching step, a mask, i.e., a second mark, 41 for use inperforming plasma etching is formed on the reverse side 31 b of thewafer 31. The mask 41 is formed to expose those areas of the reverseside 31 b of the wafer 31 that correspond to the streets 33, i.e., theareas overlapping the streets 33, and to cover those areas of thereverse side 31 b of the wafer 31 that correspond to the devices 35,i.e., the areas overlapping the devices 35. The mask 41 is made of thesame material and formed by the same process as with the mask 23 (seeFIG. 3B).

Next, a gas in a plasma state is supplied to those areas of the reverseside 31 b of the wafer 31 that are exposed and correspond to the streets33, thereby etching the areas under predetermined conditions. The wafer31 may be etched using the plasma processing apparatus 10 (see FIG. 4 ).The procedure for performing plasma etching on the wafer 31 with theplasma processing apparatus 10 is the same as the measurement etchingstep (see FIG. 5 ). The gas 90 in the plasma state is supplied to thoseareas, i.e., second areas, 31 c of the reverse side 31 b of the wafer 31that are not covered with the mask 41. The areas 31 c are exposedthrough the mask 41 and correspond to the areas corresponding to thestreets 13, i.e., the areas overlapping the streets 33.

In the etching step, the gas 90 is supplied to the wafer 31 while theprotective member 37 is disposed on the surface, i.e., the face side 31a, of the wafer 31 that is opposite the surface thereof, i.e., thereverse side 31 b, to which the gas 90 is supplied. The areas 31 c ofthe wafer 31 are etched, forming grooves in the wafer 31 from thereverse side 31 b toward the face side 31 a. When the plasma etching iscontinued until the grooves from the reverse side 31 b reach the faceside 31 a, the areas 31 c of the wafer 31 are removed. FIG. 11Billustrates the wafer 31 after the plasma etching in enlargedfragmentary cross section. The wafer 31 is thus divided along thestreets 33, producing a plurality of device chips 43 including therespective devices 35.

When the above etching step is carried out, the wafer 31 is thicker inthe areas, i.e., the central portion, where the etching rate is higherand thinner in the areas, i.e. the outer circumferential portion, wherethe etching rate is lower (see FIG. 10A). Consequently, the differencebetween the times when the grooves reach the face side 31 a of the wafer31 in the central and outer circumferential portions of the wafer 31 isreduced. The central portion of the wafer 31 is thus prevented frombeing excessively etched, and the wafer 31 is appropriately divided inits entirety.

In the method of processing a wafer according to the present embodiment,as described above, the thickness of the wafer 31 is adjusted dependingon the depths of the grooves 11 d formed in the wafer 11 by plasmaetching, and thereafter plasma etching is performed on the wafer 31. Theetching rate variations in the wafer 31 are thus reflected in thethickness distribution of the wafer 31, thereby synchronizing the timeswhen the division of the wafer 31 in the areas thereon is completed. Inthis manner, the wafer 31 is less liable to have areas where it isetched imperfectly and areas where it is etched excessively, and hencecan be divided properly.

In the above embodiment, when plasma etching is performed on the wafer11, the mask 23 is formed on the reverse side 11 b of the wafer 11 (seeFIG. 3B). However, the mask 23 may be formed on the face side 11 a ofthe wafer 11. In this case, the protective member 17 is disposed on thereverse side 11 b of the wafer 11, and the gas 90 in the plasma state issupplied through the mask 23 to the face side 11 a of the wafer 11.Similarly, when plasma etching is performed on the wafer 31, the mask 41(see FIG. 11A) may be formed on the face side 11 a of the wafer 11. Inthis case, the protective member 37 is disposed on the reverse side 31 bof the wafer 31, and the gas 90 in the plasma state is supplied throughthe mask 41 to the face side 31 a of the wafer 31.

Other changes and modifications may be made in the structural details,the method details, etc. according to the above embodiment withoutdeparting from the scope of the object of the present invention.

The present invention is not limited to the details of the abovedescribed preferred embodiment. The scope of the invention is defined bythe appended claims and all changes and modifications as fall within theequivalence of the scope of the claims are therefore to be embraced bythe invention.

What is claimed is:
 1. A method of processing a wafer, comprising: awafer preparing step of preparing a measurement wafer and a productwafer each including a first surface that has a plurality of areasdemarcated by a plurality of streets thereon and a second surface thatis opposite the first surface; a measurement etching step of forming afirst mask on the first surface or the second surface of the measurementwafer and supplying a gas in a plasma state to first areas of themeasurement wafer that are exposed through the first mask and thatcorrespond to the streets to etch the first areas under predeterminedconditions to form grooves in the measurement wafer; after themeasurement etching step, a measuring step of demarcating a plurality ofconcentric areas in an array from a center to an outer circumference ofthe measurement wafer and measuring depths of the grooves in therespective concentric areas; after the measuring step, a thicknessadjusting step of adjusting a thickness of the product wafer such thatthe product wafer is progressively thinner in areas thereof thatcorrespond to the areas of the measurement wafer where the grooves areshallower; and, after the thickness adjusting step, an etching step offorming a second mask on the first surface or the second surface of theproduct wafer and supplying a gas in a plasma state to second areas ofthe product wafer that are exposed through the second mask and thatcorrespond to the streets to etch the second areas under predeterminedconditions.
 2. The method of processing a wafer according to claim 1,wherein the thickness adjusting step includes performing grinding,polishing, or plasma etching on the product wafer to adjust thethickness of the product wafer.
 3. The method of processing a waferaccording to claim 1, wherein the measurement etching step includessupplying the gas in the plasma state to the measurement wafer while afirst protective member for protecting the measurement wafer is disposedon a surface of the measurement wafer that is opposite to the surfacethereof to which the gas in the plasma state is supplied, and theetching step includes supplying the gas in the plasma state to theproduct wafer while a second protective member for protecting theproduct wafer is disposed on a surface of the product wafer that isopposite to the surface thereof to which the gas in the plasma state issupplied.